A detailed examination will be made of host-tumor interaction in the SJL/J mouse model of transplantable and spontaneous reticulum cell sarcoma (RCS). This is said to be a model of human Hodgkin's disease and has characteristics which might help understand some aspects of certain human B cell lymphomas. We will further characterize the nature of the tumor cell, which our results indicate to be a pre-B cell, and analyze the different kinds of lymphokines that are elaborated in vitro and in vivo upon interaction of RCS cells with normal syngeneic lymph node cells. Of particular interest is the high proliferative response shown by syngeneic Lyl T cells in respnse to this tumor and the possibility, as suggested by studies in F1 hybrid mice, that presence of this response helps RCS growth in vivo. Our hypothesis is that interaction between host and tumor cells, resulting in cell proliferation and lymphokine production, are determining factors in regulation of RCS growth. Absence of growth in X-irradiated mice and reduced growth in thymectomizxed mice will be investigated from this standpoint. Possible similarities with lymphomas induced by graft versus host reactions in other mouse strains and with spontaneous lymphomas in autoimmune mice will also be investigated.